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.TH PTHREAD_KEY_CREATE 3C "Nov 2, 2007"
.SH NAME
pthread_key_create, pthread_key_create_once_np \- create thread-specific data
key
.SH SYNOPSIS
.LP
.nf
cc -mt [ \fIflag\fR... ] \fIfile\fR... -lpthread [ \fIlibrary\fR... ]
#include <pthread.h>

\fBint\fR \fBpthread_key_create\fR(\fBpthread_key_t *\fR\fIkey\fR,
     \fBvoid\fR (*\fIdestructor\fR)(\fBvoid*\fR));
.fi

.LP
.nf
\fBint\fR \fBpthread_key_create_once_np\fR(\fBpthread_key_t *\fR\fIkey\fR,
     \fBvoid\fR (*\fIdestructor\fR)(\fBvoid*\fR));
.fi

.SH DESCRIPTION
.sp
.LP
The \fBpthread_key_create()\fR function creates a thread-specific data key
visible to all threads in the process. Key values provided by
\fBpthread_key_create()\fR are opaque objects used to locate thread-specific
data. Although the same key value may be used by different threads, the values
bound to the key by \fBpthread_setspecific()\fR are maintained on a per-thread
basis and persist for the life of the calling thread.
.sp
.LP
Upon key creation, the value  \fINULL\fR is associated with the new key in all
active threads. Upon thread creation, the value  \fINULL\fR is associated with
all defined keys in the new thread.
.sp
.LP
An optional destructor function may be associated with each key value. At
thread exit, if a key value has a non-\fINULL\fR  destructor pointer, and the
thread has a non-\fINULL\fR value associated with that key, the function
pointed to is called with the current associated value as its sole argument.
Destructors can be called in any order.
.sp
.LP
If, after all the destructors have been  called for all keys with
non-\fINULL\fR values,  there are still some keys with non-\fINULL\fR values,
the process will be repeated. If, after at least
\fBPTHREAD_DESTRUCTOR_ITERATIONS\fR iterations of destructor calls for
outstanding non-\fINULL\fR values, there are still some keys with
non-\fINULL\fR values, the process is continued, even though this might result
in an infinite loop.
.sp
.LP
An exiting thread runs with all signals blocked. All thread termination
functions, including thread-specific data destructor functions, are called with
all signals blocked.
.sp
.LP
The \fBpthread_key_create_once_np()\fR function is identical to the
\fBpthread_key_create()\fR function except that the key referred to by
*\fIkey\fR must be statically initialized with the value
\fBPTHREAD_ONCE_KEY_NP\fR before calling \fBpthread_key_create_once_np()\fR,
and the key is created exactly once. This function call is equivalent to using
\fBpthread_once\fR(3C) to call a onetime initialization function that calls
\fBpthread_key_create()\fR to create the data key.
.SH RETURN VALUES
.sp
.LP
If successful, the \fBpthread_key_create()\fR and
\fBpthread_key_create_once_np()\fR functions store the newly created key value
at *\fIkey\fR and return \fB0\fR. Otherwise, an error number is returned to
indicate the error.
.SH ERRORS
.sp
.LP
The  \fBpthread_key_create()\fR and \fBpthread_key_create_once_np()\fR
functions will fail if:
.sp
.ne 2
.na
\fB\fBEAGAIN\fR\fR
.ad
.RS 10n
The system lacked the necessary resources to create another thread-specific
data key, or the system-imposed limit on the total number of keys per process
\fBPTHREAD_KEYS_MAX\fR has been exceeded.
.RE

.sp
.ne 2
.na
\fB\fBENOMEM\fR\fR
.ad
.RS 10n
Insufficient memory exists to create the key.
.RE

.sp
.LP
The \fBpthread_key_create()\fR and \fBpthread_key_create_once_np()\fR functions
will not return an error value of \fBEINTR\fR.
.SH EXAMPLES
.LP
\fBExample 1 \fRCall thread-specific data in the function from more than one
thread without special initialization.
.sp
.LP
In the following example, the thread-specific data in the function can be
called from more than one thread without special initialization. For each
argument passed to the executable, a thread is created and privately bound to
the string-value of that argument.

.sp
.in +2
.nf
/* cc -mt thisfile.c */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>

static void *thread_function(void *);
static void show_tsd(void);
static void cleanup(void*);

#define MAX_THREADS 20

static pthread_key_t tsd_key = PTHREAD_ONCE_KEY_NP;

int
main(int argc, char *argv[])
{
     pthread_t tid[MAX_THREADS];
     int num_threads;
     int i;

     if ((num_threads = argc - 1) > MAX_THREADS)
          num_threads = MAX_THREADS;
     for (i = 0; i < num_threads; i++)
          pthread_create(&tid[i], NULL, thread_function, argv[i+1]);
     for (i = 0; i < num_threads; i++)
          pthread_join(tid[i], NULL);
     return (0);
}

static void *
thread_function(void *arg)
{
     char *data;

     pthread_key_create_once_np(&tsd_key, cleanup);
     data = malloc(strlen(arg) + 1);
     strcpy(data, arg);
     pthread_setspecific(tsd_key, data);
     show_tsd();
     return (NULL);
}

static void
show_tsd()
{
     void *tsd = pthread_getspecific(tsd_key);

     printf("tsd for %d = %s\en", pthread_self(), (char *)tsd);
}

/* application-specific clean-up function */
static void
cleanup(void *tsd)
{
     printf("freeing tsd for %d = %s\en", pthread_self(), (char *)tsd);
     free(tsd);
}
.fi
.in -2

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Committed.
_
MT-Level	MT-Safe
_
Standard	See below.
.TE

.sp
.LP
For \fBpthread_key_create()\fR, see \fBstandards\fR(7).
.SH SEE ALSO
.sp
.LP
.BR pthread_getspecific (3C),
.BR pthread_key_delete (3C),
.BR pthread_once (3C),
.BR pthread_setspecific (3C),
.BR attributes (7),
.BR standards (7)
